Some subway tunnels are V-shaped slope tunnels due to the need of crossing tunnel, pipeline, geological conditions and tunnel drainage. Smoke competition in V-shaped slope tunnel will increase the complexity and difficulty of smoke control in subway tunnel fires. The influences of tunnel slope, tunnel slope and height difference of V-shaped slope tunnel on critical velocity and smoke back-layering length in subway tunnel fires are investigated by using the theoretical analysis method and numerical simulation method. Results showed that tunnel slope and V-shaped slope had significant effects on critical velocity and smoke back-layering length in subway tunnel fires. The critical velocity decreased with the increasing of the tunnel slope for the uphill tunnels . The critical velocity increased with the increasing of the tunnel slope for the downhill tunnels . The critical velocity decreased with the increasing of the tunnel slope for the uphill tunnels of V-shaped slope tunnels. The critical velocity decreased with the increasing of the height difference for the downhill tunnels of V-shaped slope tunnels. Research results can provide the theoretical basis and technical support for determining the critical velocity in the ventilation design and smoke control system design for V-shaped slope subway tunnels.